(1) Field of the Invention
This invention relates to a variable displacement compressor, and more particularly to a variable displacement compressor for use in compressing a refrigerant gas in a refrigeration cycle of an automotive air conditioner.
(2) Description of the Related Art
A compressor used for compressing refrigerant in a refrigeration cycle of an automotive air conditioner is driven by an engine, and hence the rotational speed of the compressor cannot be controlled. For this reason, a variable displacement compressor capable of changing the capacity of refrigerant to be compressed is employed so as to obtain adequate cooling power without constraints of the rotational speed of the engine.
In such a variable displacement compressor, compression pistons are connected to a wobble plate fitted on a shaft driven for rotation by the engine, and the angle of the wobble plate is changed to vary the length of piston stroke, whereby the discharge capacity of refrigerant is changed.
The angle of the wobble plate is continuously changed by introducing part of the compressed refrigerant into a gastight crank chamber and changing the pressure of the introduced refrigerant, thereby changing a balance between pressures applied to the both ends of each piston.
The variable displacement compressor has a solenoid control valve arranged between a discharge port for delivering refrigerant and the crank chamber or between the crank chamber and a suction port. This solenoid control valve opens and closes the communication such that the differential pressure across the solenoid control valve is maintained at a predetermined value. The predetermined value of the differential pressure can be externally set by a current value. Due to this configuration, when the engine rotational speed increases, the pressure introduced into the crank chamber is increased to reduce the capacity for compression, while when the engine rotational speed decreases, the pressure introduced into the crank chamber is reduced to increase the capacity for compression, whereby the pressure of refrigerant discharged from the compressor is maintained at a constant level.
Although a chlorofluorocarbon substitute HFC-134a is generally used as a refrigerant in a refrigeration cycle of an automotive air conditioner, there has recently been developed a refrigeration cycle which causes refrigerant to perform refrigeration in a supercritical region where the temperature of the refrigerant is above its critical temperature, e.g. a refrigeration cycle using carbon dioxide as refrigerant.
However, in the solenoid control valve for controlling the pressure introduced into the crank chamber according to the discharge pressure of the compressor, in the case of the refrigeration cycle using carbon dioxide as the refrigerant, since the pressure of the refrigerant is increased to the supercritical region, the differential pressure between the discharge port for delivering the refrigerant and the crank chamber or between the discharge port and the suction port becomes very large, and hence a solenoid force for controlling the differential pressure also becomes very large. This necessitates a large-sized solenoid, causing an increase in the size of the solenoid control valve, which results in increased manufacturing costs.